Stable and rigorous simulation for all-way coupled multiphase flow and geomechanics in a bi-wing hydraulically fractured system: The vacuum area and dry zone

Abstract

We investigate the effects of coupled multiphase flow and geomechanics in the hydraulically fractured reservoirs numerically. We account for poromechanics of flow within a discrete fracture for rigorous simulation, considering both fluid compressibility and fracture volume change. We newly propose a numerically stable sequential method for all-way coupled geomechanics and flow for the discrete fractured systems. The stabilization term is dynamic, depending on the fracture length, and the numerical results support good stability and convergence of the proposed sequential method for hydraulic fracturing simulation. From numerical experiments, we identify at the fracture tip the vacuum area where pressure is below the initial reservoir pressure as well as dry zone where gas saturation is high. The existence of vacuum area is fundamentally due to different time scales between flow and geomechanics, which can be considered as the Mandel-Cryer effect in the fractured systems. The dry zone is not necessarily identical to the vacuum area. The reservoir gas can be introduced into the fracture substantially when the reservoir is highly permeable. The vacuum area and the dry zone can be prominent in naturally or previously hydraulically fractured reservoirs. We study the system responses of the fracture propagation by varying parameters of flow and geomechanics. The hydraulic fracture propagates slowly when reservoir permeability or fracture toughness is high, when Young’s modulus is low, or when the initial total stress is higher than the initial reservoir pressure.